1. Field of the Invention
This invention relates to electrolytic recovery of zinc from its salts, particularly from acid sulfate solutions. Metallic zinc is deposited at the cathode while the anode is depolarized through oxidation of iodide ions to iodine, avoiding oxygen evolution at the anode. The iodide ions are chemically regenerated by the extracellular oxidation of sulfur dioxide with water to produce iodide ions and hydrogen ions for recycle to the anode compartment. The utilization of iodide ions in the depolarization of the anode results in operation of the electrolytic cell at near the decomposition potential for zinc sulfate while obtaining good deposits of elemental zinc at high faradic efficiencies. This results in energy savings of 30 to 40 percent over conventional electrowinning of zinc.
2. Description of the Prior Art
Electrolytic deposition of zinc as a method of zinc recovery from acid zinc sulfate solutions has been practiced: deposition upon lead electrodes taught by U.S. Pat. No. 1,252,654; electrodeposition of zinc on ferrous metals using an alkali salt of fluorine and aluminum chloride in the electrolyte is taught by U.S. Pat. No. 2,446,983 and plating of ferrous metals with zinc for the avoidance of corrosion from an electrolyte containing ammonium chloride, aluminum sulfate and sodium fluoride followed by heat treatment of the coated material is taught by U.S. Pat. No. 3,254,005; and electrolytic deposition of zinc on aluminum from a zinc pyrophosphate plating bath is taught by U.S. Pat. No. 2,511,952 and electroplating of zinc on aluminum using a plating bath with fluotitanate or fluozirconate ion containing plating bath is taught by U.S. Pat. No. 3,079,310.
U.S. Pat. No. 4,059,596 teaches a process for the electrolytic production of sulfuric acid by oxidation of sulfur dioxide in the anode compartment of an electrolysis cell and producing hydrogen in the cathode compartment and utilizing hydrogen iodide or iodine in the anode compartment with sulfur dioxide and water. The '596 patent recognizes the lowering of overvoltage with the use of hydrogen iodide or iodine in the anode compartment for the electrolytic oxidation of sulfur dioxide.